Using PHP to reload a page w/variable.

I'm finding it hard to believe this is as difficult as it appears, but everywhere I search indicates that what I want to do is not an easy task. How do you code this? You have a user signup page and the user fills it out, filling out two password fields to ensure they typed it in properly and hit Submit. How in PHP do you evaluate the password strings (or other strings like checking email for a valid email address) and if something is wrong, reload the page. In my case, I have one page that does multiple things and I pass variables to determine what parts of the page load, so ideally, I'd like to reload the page and pass a variable so I can reveal some text like "Please retype your password". Is it really as difficult as some sites make it out to be? It seems to be a pretty common thing to do. Thanks!